Ground Fault Circuit Interrupter (GFCI) is a ground fault electric-leakage protection device to protect from accidents of electric shock when electricity is used at wet occasions, such as kitchen, toilet and basement, etc. in ordinary residential houses. According to the stipulation of National Electrical Code of USA, at least 8 such devices need to be mounted in an ordinary medium-sized suite, and need to be changed once every two years. In other words, the annual demand of GFCI in the United States is more than 20 million devices with a total market price of more than USD 3 billion. The safety standard of the above mentioned product must be in accordance with the authentication requirement of UL (Laboratory of Insurance Co.) in USA. The authentication organization of UL has declared that GFCI electric-leakage protection devices with self-diagnostic protection function should be widely put in use as of 28 Jun. 2006 in USA. The product has self-diagnostic function in real time, and can detect whether or not the electric-leakage protection circuit is in normal working state. Once the circuit has trouble, alarming signal in the form of sound or light, etc. will be given.
Normally, an integrated-circuit, e.g. integrated-circuit FM2145, FM2141, etc., is used as the core component of GFCI products. Now such integrated-circuit has already been widely used. But GFCI circuit by using of existing GFCI integrated-circuit is designed based upon the former UL standard, and does not comply with the application of the new standard mentioned above. Thus, the present design is required to satisfy the requirement of that new standard. Generally speaking, GFCI electric-leakage protection device with self-diagnostic function needs a circuit to produce an artificial emulation leakage current. When the emulation leakage current passes through a magnetic leakage current detecting ring, the current at the input end of the electric-leakage detecting circuit will be induced by the magnetic leakage current detecting ring, thus to trigger the electric-leakage protection circuit to handle that emulation leakage current. Each output end of the electric-leakage protection circuit will be detected by the self-diagnostic circuit to determine whether or not the electric-leakage protection circuit is working properly. Normally, the leakage current produced by that circuit will be caused at the time when GFCI tripping mechanism can not be initiated, otherwise the regular protection function will be interrupted. Therefore, the emulation leakage current is usually unidirectional in nature, i.e., it only has one direction, and the signal shall have a direct current (DC) component. But due to the addition of the emulation leakage current, when an actual leakage current is caused, the emulation leakage current will be superimposed on the actual leakage current, due to the unidirectional nature of that emulation leakage current, the influence of forward and backward leakage currents caused by the actual leakage current will be different. Accordingly, after superimposing, the magnitudes of the leakage currents at forward and backward directions will be different and that the sensitivity of GFCI product is different when measuring from different directions of the leakage currents. Therefore, it does not comply with the consistency requirement of leakage currents at forward and backward directions stipulated by the UL standard. For example, when the emulation leakage current is superimposed on backward leakage current, as a result, the sensitivity of forward leakage current will be comparatively low and the sensitivity of backward leakage current will be comparatively high.